In communication systems operating according to the LTE Advanced standards, a radio access network (RAN) provides User Equipment (UE), such as mobile telephones, access to a core network (and hence to other user equipment or other network nodes) via one or more of its cells. A radio access network typically comprises a plurality of base stations (eNB), each of which operates one or more cells of that RAN. Communication between the mobile telephones and the radio access network is controlled using a Radio Resource Control (RRC) protocol as defined in 3GPP TS 25.331 for UTRAN and TS36.331 for E-UTRAN. RRC handles the control plane signalling of Layer 3 between mobile telephones and the radio access network, and includes, inter alia, functions for broadcasting system information, paging, connection establishment and release, radio bearer establishment, reconfiguration and release, mobility procedures, and power control.
At any given time, mobile telephones may operate either in an ‘RRC idle mode’ or an ‘RRC connected mode’, the latter of which includes a ‘CELL_PCH’ (Cell Paging channel) and a ‘URA_PCH’ (URA Paging channel) modes, a ‘CELL_FACH’ (Forward access channel) mode, and a ‘CELL_DCH’ (Dedicated Channel) mode for the UTRAN access.
The radio access network controls the transition between the various operating modes for each mobile telephone within the cells of its base stations. Since the setting up and termination of an RRC connection between a base station of the RAN and the mobile telephone requires exchanging of signalling messages and hence utilises valuable system resources, and also takes some time to complete, the transition from connected to idle mode is only allowed under specific circumstances as defined in the 3GPP TS 25.331 for UTRAN and TS36.331 for E-UTRAN standards, the contents of which are incorporated herein by reference. For example, the serving base station (eNodeB/eNB for E-UTRAN and NodeB for UTRAN) might instruct a mobile telephone to enter the RRC idle mode only after it has confirmed that there is no more data to be transmitted to/from the particular mobile telephone (e.g. both uplink and downlink buffers are empty).
In particular, RRC protocol provides inactivity timers to control transitions to lower energy consuming states (i.e. when no data is transmitted within a certain time period), thereby preserving battery life of the mobile telephones whenever possible whilst also ensuring that the transition to idle mode does not happen too soon. In UTRAN for example, a so-called ‘T1’ timer controls the mobile telephone's transition from DCH to FACH mode, a ‘T2’ timer controls transition from FACH to PCH mode, and a ‘T3’ timer controls transition from PCH to idle mode. Different inactivity timer values can be set and broadcast by the radio access network, which result in different overall energy consumption of the mobile telephones (both active and idle) served by the base stations of that RAN.
For mobile telephones operating in the RRC connected mode, the RAN (e.g. a base station in case of GSM EDGE RAN (GERAN), a Radio Network Controller (RNC) in case of UTRAN, or an eNB in case of E-UTRAN) may optimise power consumption by configuring a so-called Discontinuous Reception (DRX) and/or Discontinuous Transmission (DTX) operation. Both techniques are based on reducing the mobile telephone's transceiver duty cycle while in active operation.
In DRX mode, the RAN sets a cycle during which the mobile telephone is operational for a certain period of time and the RAN transmits all scheduling and paging information (for this mobile telephone) during this period only. The mobile telephone can thus turn off its transceiver for the rest of the DRX cycle. DRX also applies to the RRC idle mode with a longer cycle time than in connected mode.
In DTX mode, the mobile telephone does not turn off its transceiver completely, but keeps monitoring the Physical Downlink Control Channel (PDCCH) to be able to receive data from the base station without undue delay.
The longer the ‘off’ duration relative to the duty cycle, the more power saving can be achieved.
A so-called System Frame (SF) is the largest time interval within the frame structure of UTRAN and E-UTRAN which can be used for synchronization between the RAN and the mobile telephone. Each radio frame within the SF is associated with a ‘relative’ frame number from #0 to #n−1 (where ‘n’ is the number of frames in the SF). This radio frame number (or frame index) is also referred to as System Frame Number (SFN). In E-UTRAN and UTRAN networks, a DRX cycle can be scheduled based on the SFN of the UTRAN Paging Indication Channel (PICH) or the E-UTRAN Physical Downlink Control Channel (PDCCH).
In the current 3GPP specifications the maximum length of the DRX cycle is less than the length of the SF, as it is limited to 5.12 s and 2.56 s for UTRAN and E-UTRAN, respectively. When a node in the RAN (e.g. base station, NodeB, eNB, etc.) needs to send a paging message to the mobile telephone, it calculates the timing of the paging message (i.e. the radio frame or sub-frame in which the paging message is to be sent) for the target mobile telephone by taking into account, amongst other things, the DRX cycle length currently applied for that mobile telephone. Paging messages are sent only in those radio frames in which the mobile telephone is known to operate its transceiver, in accordance with its DRX cycle.